Reading and wiping notes also adding more notes

[Joel]

I'm trying to add a feature to my text modifying program. I take 
a lot of notes but I want to be able to both wipe what I've read 
while still adding more notes as I go.

I have two text fields. The one on the left has the whole text, 
new stuff being added to the bottom. The one on the right has 
text I've been wiping as I'm reading. I want to be able to add 
from the whole text just the new stuff at the bottom to the right 
text at the bottom. - I hope I'm making sense (good exercise, 
trying to explain it helps me work it out by itself).

Example
Whole text: [I went for a walk and fell down a hole. There was a 
D guy on the roof. That was a tricky problem!]

Read and wipe text: [There was a d guy on the roof.]

Now I want to add "That was a tricky problem!" to the wipe text 
to get: [There was a D guy on the roof. That was a tricky 
problem!]